In a diesel engine provided with a DPF (diesel particulate filter) for removing particulates (particulate matters (hereinafter referred to as PM)) in an exhaust gas of the diesel engine, the DPF comes in two types, namely, a metal type and a ceramic type. The metal type is easy to handle, but has a low PM trapping efficiency of 50% to 60%. Meanwhile, the ceramic type has a high trapping efficiency of 90% or more, but tends to allow PM to easily accumulate in the DPF, requiring PM be forcibly burnt.
In the future, a PM regulation value is expected to be stricter, so that the ceramic type with high trapping efficiency is considered to be the mainstream. To regenerate the DPF, it will be essential to accurately estimate a PM accumulation amount on the basis of the operation history of an engine or a condition of the DPF.
FIG. 6 is a longitudinal sectional diagram of a typical DPF apparatus. In FIG. 6, reference numeral 13 denotes an exhaust pipe in communication with an exhaust port of an engine (not shown), and reference numeral 50 denotes the DPF apparatus connected to the exhaust pipe 13. The DPF apparatus 50 includes a DPF 1 accommodated in a DPF main unit 2 and a pre-stage oxidation catalyst 3 installed on the upstream side of the DPF 1.
The exhaust gas from the engine passes through an inlet chamber 4 from the exhaust pipe 13 and reaches the pre-stage oxidation catalyst 3, and then the exhaust gas is oxidized by the pre-stage oxidation catalyst 3. The oxidative heat generated at that time causes the DPF 1 to rise to 600° C. to 650° C. to burn the PM built up in the DPF 1, and the combustion gas is exhausted to the outside through an outlet chamber 5.
Referring to FIG. 6, P1 denotes an inlet pressure of the DPF 1, T1 denotes an inlet temperature of the DPF 1, P2 denotes an outlet pressure of the DPF 1, and T2 denotes an outlet temperature of the DPF 1.
FIG. 7 is a block diagram of the estimation of a PM accumulation amount in a regeneration controller of a conventionally used DPF.
In the figure, the value of an engine PM emission amount schematically calculated as a model value has been set in a PM emission amount model 11 in the form of an estimation map, the value being schematically calculated on the basis of the engine speed, a target fuel injection amount, and the opening degree of a throttle valve of an engine, and also the opening degree of an EGR valve, which controls the amount of EGR, if the engine is an EGR (exhaust gas recirculation) type engine.
Further, the value of a PM regeneration amount in the DPF 1 schematically calculated as a model value on the basis of an actually measured value T1 of the inlet temperature T1 of the DPF 1 and an outlet temperature T2 of the DPF 1 has been set in the PM regeneration amount model 12 in the form of an estimation map.
Then, the PM regeneration amount model 12 is subtracted from the PM emission amount model 11 to determine an estimated PM amount estimation logic 14a. 
Meanwhile, the value of an exhaust gas flow rate schematically calculated as a model value has been set in an exhaust gas flow rate model 13 in the form of an estimation map, the value being calculated on the basis of the engine speed, a target fuel injection amount, and the opening degree of a throttle valve of an engine, and also the opening degree of the EGR valve, which controls the amount of EGR, if the engine is the EGR type engine.
Further, as the detection value of a differential pressure of the DPF 1, the differential pressure between the outlet pressure P2 of the DPF 1 and the inlet pressure P1 of the DPF 1 has been calculated as the DPF differential pressure.
Further, the DPF differential pressure schematically calculated as a model value by the exhaust gas flow rate model 13 and the DPF differential pressure is set as a DPF differential pressure model 15 in the form of an estimation map.
Thus, in a PM accumulation amount estimator 17a, the estimated PM accumulation amount, which is the total amount of PM, is determined by the PM accumulation amount estimation logic 14a, which has been estimated, and the DPF differential pressure model 15.
Further, the integrated value of the fuel injection amount calculated on the basis of the engine speed and the target fuel injection amount of the engine, and time by a fuel consumption amount integrating meter 16, is stored in the PM accumulation amount estimator 17a. 
As described above, according to the prior art, if the three set estimation elements, namely, the PM accumulation amount estimation logic 14a, the DPF differential pressure model 15, and the fuel consumption amount integrating meter 16, exceed reference upper limit values of accumulation amount, then it means that the PM accumulation amount has reached the limit thereof.
Further, according to the one disclosed in Patent Document 1 (Japanese Patent Application Laid-Open No. 2004-197722), a first PM accumulation amount PM1 is estimated on the basis of a differential pressure before and after a DPF, a PM emission amount is integrated to calculate a second PM accumulation amount PM2, and the PM1 or the PM2, whichever is larger is adopted as the PM accumulation amount which should be the final estimated value.
In the DPF accumulation amount estimating apparatus illustrated in FIG. 7, as described above, the three estimation elements, namely, the set PM accumulation amount estimation logic 14a, the DPF differential pressure model 15, and the fuel consumption amount integrating meter 16, are independently constructed, and if the accumulation amount upper limits on which the aforesaid three elements are based are exceeded, then it means that the PM accumulation amount has reached the limit thereof.
For this reason, if an actual PM emission amount exceeds the aforesaid estimation elements, then the estimated PM amount output by the PM accumulation amount estimation logic 14a will significantly deviate from an actual accumulation amount as time elapses, and present a problem especially if the actual accumulation amount becomes the estimated PM amount or more.
Further, in the DPF accumulation amount estimating apparatus illustrated in FIG. 7, the DPF differential pressure model 15 is considerably influenced by an exhaust gas flow rate, so that the exhaust gas flow rate need to be introduced into an estimation model. However, it is difficult to measure the relationship among the DPF differential pressure, the exhaust gas flow rate, and the PM accumulation amount over the entire engine operation range with high accuracy. Hence, depending on the operating condition of an engine, the estimation accuracy of the DPF differential pressure model 15 may deteriorate.
Especially in the case where the operating condition of an engine significantly changes or in an operating condition of a low load, the PM accumulation amount estimated from the PM accumulation amount estimation logic 14a and the DPF differential pressure model 15 may deviate from an actual accumulation amount.